1. Field of the Invention
The present invention relates to a control device for controlling an induction motor which drives an electric vehicle such as an electromotive railcar.
2. Description of the Related Art
FIG. 9 shows a conventional electric vehicle control device 1 for controlling an induction motor 53 which drives an electric vehicle such as an electromotive railcar.
A command controller 42 transforms a torque command sent by a motorman's cab controller 47 to a control signal 50 and sends the control signal to a VVVF (Variable Voltage and Variable Frequency) inverter 52. VVVF inverter 52 receives direct current (DC) power collected from an external power line via a pantogragh 51 and converts the DC power to a VVVF alternating current (AC) power in order to drive the induction motor 53.
At this time, the AC power is transformed into kinetic energy and core loss (iron loss and copper loss). The loss causes a heat rise in the induction motor. In general, a cruising pattern between stops is composed of power running (acceleration), coasting, regenerative brake, and stop stages. This pattern is repeated. In this case, the induction motor is provided AC power only during the power running and regenerative brake stages. The AC power is not supplied during either the coasting or stop stages.
Moreover, since the induction motor is installed on a truck or bogie of electromotive railcar, the external form is restricted and users require further miniaturization and lighter weight. Therefore, the induction motor 53 is designed and produced with careful consideration to wind generated while coasting, cruising and stopping.
However, as the driving operation extends for a long time, the induction motor 53 becomes covered with dust due to the wind directed for cooling the induction motor 53. Further, according to driving conditions, the duration of the power running (acceleration), regenerative brake, coasting and stop stages changes. Consequently, a three-phase induction motor 53 is not cooled sufficiently, which causes a dielectric breakdown and burning in coils or the core of induction motor 53.
Moreover, since the induction motor installed on a truck of an electromotive railcar is required to be both restricted in external form and durable in extremely severe conditions, it is difficult to install a sensor such as a thermometer. Even if a sensor is set, it is very expensive.
Therefore, there is a possibility that the previous model of the control device causes a dielectric breakdown and burning, when temperature increases abnormally.